Chapter 8: Long-term Phytoplankton Community Dynamics: Oneida Lake (1975-2011)
Nasseer Idrisi, Edward L. Mills, and Lars G. Rudstam
The structure of phytoplankton assemblages is regulated by both bottom-up and top-down processes (Carpenter and Kitchell 1984; McQueen et al. 1986; Persson et al. 1992). These regulatory processes have been shown through manipulative experimentation in lake ecosystems (McQueen et al. 1989; Carpenter and Kitchell 1993; Carpenter et al. 2011). However, manipulative experiments are an exaggeration of the natural system designed to demonstrate mechanism and process. Identifying processes that regulate ecosystem structure under natural conditions is difficult due to high temporal and spatial variability (Currie et al. 1999). Long-term data series may help identify patterns in natural systems when variability is high, but long-term data series are rare, and they often encompass anthropogenic and natural perturbations that change the ecosystem in unexpected ways (Edmondson 1970; Mills et al. 1993; Strayer et al. 1999; Idrisi et al. 2001; Hampton et al. 2006). For example, the 1970s ban on phosphates in detergents as mandated by the Great Lakes Water Quality Agreement led to a reduction in the concentrations of phosphorus in the Oneida Lake ecosystem. As a result, phosphorus concentrations in the lake declined from high values exceeding 100 μg/L in the 1960s to the current levels of 20–40 μg/L (Zhu et al. 2006; Fitzgerald et al. Chapter 10). We expect that this reduction in nutrient levels changed the phytoplankton community structure and seasonal succession from the bottom-up (Idrisi et al. 2001). At the same time, changes in grazing by herbivorous zooplankton and the invasive zebra mussel Dreissena polymorpha may have reduced phytoplankton standing crop (Carpenter and Kitchell 1993; Idrisi et al. 2001; Higgins and Vander Zanden 2010). These bottom-up and top-down forces likely interacted to affect both standing crop and phytoplankton community structure in the Oneida Lake pelagic ecosystem over ecological time.